Quantification of the effect of physical separation of pigs on spread of Streptococcus suis

Streptococcus suis (S. suis) infections are common in swine herds, resulting in abundant use of antibiotics and economic damage. Control of the disease might be achieved by reduction of spread of S. suis by physical separation of groups of pigs. The aim of this study was to quantify transmission of S. suis among pigs which had either direct or indirect contact with each other. In three replicate experiments, pigs were housed in boxes either pair-wise (direct transmission; N=50) or alone (indirect transmission; N=15). Stable units contained 2-4 pairs, and 1-3 single housed pigs. The distance between the boxes was ± 1 m. At 7 weeks of age, one pig in each pair was inoculated intranasally with S. suis serotype 9. Colonization was monitored in all pigs for 4 weeks post inoculation by taking tonsillar brush and saliva swab samples that were cultured on selective media. All directly exposed pigs became colonized within 2 days after exposure. Thirteen indirectly exposed pigs became positive within 7-25 days after exposure. For direct transmission, a standard SIR model was used to estimate the transmission rate βdir (3.58, 95% CI: 2.29-5.60). For indirect transmission, a model with linearly increasing infectivity of infectious animals (representing, for instance, accumulation of S. suis in the environment) fitted better than the standard model. The results show that physical separation of an infectious and a susceptible pig over ± 1 meter results in a mean time to infection of 39 days (95% CI: 30-51), compared to 0.28 days (95% CI: 0.18-0.44)  when housed together. We conclude that keeping pigs separated might be an effective intervention measure to reduce horizontal transmission of S. suis within stables.